Normally the cook and medic designations are separate duties on a ship. However, they can be joined if the person has the aptitude. In a fantasy setting like D&D, you can easily have a person be a healer class with Craft(Cooking) as a skill.
Example: Cynthia, Healer 3 "Ship Doctor's Assistant" Feats: Augment Healing, Brew Potion Skills ranks: Concentration 1, Healing 5, Craft (Cooking) 5, Knowledge (Nature) 1, Profession (Sailor) 1, Sense Motive 1.
She has charm person, suggestion, and dominate, so there may be some opportunities to use a mind-controlled barbarian to sabotage enemy defenses, locate refugees or enemies, destroy or steal supplies, distract (or even attack) an enemy commander or wizard, and so on. Generally, strategies like these will require more planning in advance.
What's the range on Clairvoyance? She might be able to get us pre-battle information before the fleet ever reaches the coast. Detect psionics is a good plan, though it requires getting low enough for the 60ft cone to be useful. Does the control skull emit that power farther than normal (as though enlarged)?
The saboteur plan is a solid option, though we'd have to know what we're up against before that would be feasible. As a contingency, Yasheen could be prepped to dominate a barbarian at need. I could see suggestion on graith witnesses as being handy. "Forget that we were here" makes it easier to escape notice than having to kill and hide a body. Could clairvoyance help give us some clues as to who watch out for, and can you cast detection spells through your sensor?
The cook(s) could be a healer class with brew potion and spend much of the voyage brewing potions and making food. Perhaps that is a way Githyanki make their graith useful, doubling as medics as food providers. Who ever heard of a Gith complaining about their food by saying it tastes like medicine?
I could even see the armorer doubling as the ship's alchemist, crafting alchemical weapons as well as maintaining the ship's siegecraft and cannons.